1. Field of the Invention
This invention relates to methods and apparatus for making coated articles with one or more layers by dip, spray or flow coating. In one aspect, this invention relates to an apparatus and method for making coated containers, preferably comprising polyethylene terephthalate, from coated preforms.
2. Description of the Related Art
Preforms are the products from which containers are made by blow molding. Unless otherwise indicated the term “container” is a broad term and is used in its ordinary sense and includes, without limitation, both the preform and bottle container therefrom. A number of plastic and other materials have been used for containers and many are quite suitable. Some products such as carbonated beverages and foodstuffs need a container, which is resistant to the transfer of gases such as carbon dioxide and oxygen. Coating of such containers has been suggested for many years. A resin now widely used in the container industry is polyethylene terephthalate (PET), by which term we include not only the homopolymer formed by the polycondensation of [beta]-hydroxyethyl terephthalate but also copolyesters containing minor amounts of units derived from other glycols or diacids, for example isophthalate copolymers.
The manufacture of biaxially oriented PET containers is well known in the art. Biaxially oriented PET containers are strong and have good resistance to creep. Containers of relatively thin wall and light weight can be produced that are capable of withstanding, without undue distortion over the desired shelf life, the pressures exerted by carbonated liquids, particularly beverages such as soft drinks, including colas, and beer.
Thin-walled PET containers are permeable to some extent to gases such as carbon dioxide and oxygen and hence permit loss of pressurizing carbon dioxide and ingress of oxygen which may affect the flavor and quality of the bottle contents. In one method of commercial operation, preforms are made by injection molding and then blown into bottles. In the commercial two-liter size, a shelf life of 12 to 16 weeks can be expected but for smaller bottles, such as half liter, the larger surface-to-volume ratio severely restricts shelf life. Carbonated beverages can be pressured to 4.5 volumes of gas but if this pressure falls below acceptable product specific levels, the product is considered unsatisfactory.
It is therefore desirable to provide the container with a layer of a barrier material which has a low vapor and gas permeability. Barrier layers may be provided by a variety of techniques, including coinjection, chemical vapor deposition, plasma coating with amorphous carbon and/or SiOx, etc., so as to form a laminar coated container. Other examples involve the use of an aqueous dispersion of barrier polymers, and have included dispersions made from vinylidene chloride with acrylonitrile and/or methyl acrylate, optionally containing units derived from other monomers such as methyl methacrylate, vinyl chloride, acrylic acid, or itaconic acid, dispersions made from EVOH and MXD6, etc. The dispersions typically contained surfactants such as sodium alkyl sulphonates.